Thermal engines play an indispensible role in everyday life. Environmental concerns have urged a need to design thermal engines which are more environmentally friendly, highly efficient and cost effective. The heat required by thermal engines may be provided from combustion of fuel, geothermal sources, solar radiation, or any other available heat source.
A thermal engine converts thermal energy captured from a heat source into mechanical energy, which can be either utilized directly to drive a mechanical device or further converted to electricity via a generator. A thermal engine may be either a piston engine or a turbine engine.
A piston engine comprises at least a cylinder, a piston, a crankshaft, and a working fluid. Generally, the working fluid undergoes thermodynamic cycles in the cylinder chamber, which drives the piston to move inside the respective cylinder, transmitting the resulting mechanical power through the crankshaft.
One of the efficiency-determining factors of a piston engine is the admission and exhaust of the working fluid into and out of the cylinder chamber. In most piston engines, the admission and exhaust processes are controlled by poppet valves. The dead space created by the position and configuration of the poppet valves and intake/exhaust ports is a major contribution to the low efficiency of piston engines.
Additionally, there are several other disadvantages associated with poppet valves: 1) the flow forces of the working fluid act directly in the direction of poppet motion, which creates an unbalanced force on the valve and makes its dynamic control difficult; 2) the poppet displacement to port opening area ratio is large, thus requiring very high resolution and high bandwidth poppet position control to maintain fine flow regulation; and 3) the design of a poppet valve is specific to the cylinder and port configuration of the engine. Thus, it is difficult for one valve design to adapt to different cylinder and port configurations.